Giant Magnetoresistance Elements Research Articles

The Effect of Geometrical Overlap between Giant Magnetoresistance Sensor and Magnetic Flux Concentrators: A Novel Comb-Shaped Sensor for Improved Sensitivity

The combination of magnetoresistive (MR) element and magnetic flux concentrators (MFCs) offers highly sensitive magnetic field sensors. To maximize the effect of MFC, the geometrical design between the MR element and MFCs is critical. In this paper, we present simulation and experimental studies on the effect of the geometrical relationship between current-in-plane giant magnetoresistive (GMR) element and MFCs made of a NiFeCuMo film. Finite element method (FEM) simulations showed that although an overlap between the MFCs and GMR element enhances their magneto-static coupling, it can lead to a loss of magnetoresistance ratio due to a magnetic shielding effect by the MFCs. Therefore, we propose a comb-shaped GMR element with alternate notches and fins. The FEM simulations showed that the fins of the comb-shaped GMR element provide a strong magneto-static coupling with the MFCs, whereas the electric current is confined within the main body of the comb-shaped GMR element, resulting in improved sensitivity. We experimentally demonstrated a higher sensitivity of the comb-shaped GMR sensor (36.5 %/mT) than that of a conventional rectangular GMR sensor (28 %/mT).

Detection of Surface and Subsurface Flaws with Miniature GMR-Based Gradiometer.

The eddy-current (EC) testing method is frequently utilized in the nondestructive inspection of conductive materials. To detect the minor and complex-shaped defects on the surface and in the underlying layers of a metallic sample, a miniature eddy-current probe with high sensitivity is preferred for enhancing the signal quality and spatial resolution of the obtained eddy-current images. In this work, we propose a novel design of a miniature eddy-current probe using a giant magnetoresistance (GMR) sensor fabricated on a silicon chip. The in-house-made GMR sensor comprises two cascaded spin-valve elements in parallel with an external variable resistor to form a Wheatstone bridge. The two elements on the chip are excited by the alternating magnetic field generated by a tiny coil aligned to the position that balances the background output of the bridge sensor. In this way, the two GMR elements behave effectively as an axial gradiometer with the bottom element sensitive to the surface and near-surface defects on a conductive specimen. The performance of the EC probe is verified by the numerical simulation and the corresponding experiments with machined defects on metallic samples. With this design, the geometric characteristics of the defects are clearly visualized with a spatial resolution of about 1 mm. The results demonstrate the feasibility and superiority of the proposed miniature GMR EC probe for characterizing the small and complex-shaped defects in multilayer conductive samples.

On-chip implementation of different analog linearization schemes for giant-magnetoresistance sensors with a comparative study

Giant magnetoresistance (GMR) sensors measure the external magnetic fields using a Wheatstone bridge configuration, with two active and two passive GMR elements. The differential output of the GMR-bridge is inherently non-linear and requires an additional interfacing front-end for linearization. In this paper, three different linearizing schemes are implemented, using a standard 180-nm CMOS technology, and a comparative study is presented. These schemes show a linear voltage-field relation in their characterizations. All these schemes require a single positive supply voltage of 3 V and provide a single-ended output. Efficacies of these schemes are brought out using simulation-based studies, considering the static and the time-varying input changes. The performance comparison of these schemes shows various trade-offs, such as component-count, power-consumption, maximum error, etc., with the existing GMR linearization circuits and provides the selection guidelines based on application requirements.

A Model to Design Giant Magnetoresistive Sensor

Giant magnetoresistive (GMR) sensor has developed rapidly in automobile, consumer electronics and current sensing areas due to its high sensitivity and low power consumption. In this work, we suggest a method to design GMR sensor based on Stoner-Wohlfarth coherent rotation model. We have investigated the influence of width of GMR sensor to its performance with the model and verified with experiments. And different pinning direction GMR elements are also researched to study Wheatstone bridge structure. The experimental results matched well with the simulation results which proved that the model is efficient in designing GMR sensor.

Symmetric Response Magnetoresistance Sensor With Low 1/f Noise by Using an Antiphase AC Modulation Bridge

A new high-sensitive magnetic sensor which is referred to a symmetric response magnetoresistance (SRMR) sensor is proposed. This sensor uses giant magnetoresistance (GMR) elements which have symmetric field response. An ac magnetic field with frequency f is applied to the GMR elements in order to modulate a small detecting magnetic field. Due to the symmetric response of the MR element, a carrier signal of the ac magnetic field is converted to 2f and the detecting signal is converted to f . By using a full bridge circuit, where two pairs of GMR elements have opposite phase of the applied ac field, the 2f components can be eliminated at the differential output terminal of the bridge. Since this system converts the frequency range of the detecting magnetic field to higher one, its noise level, which is dominated by the 1/f noise, can be decreased. In this study, the SRMR sensor with the antiphase ac modulation bridge was fabricated and investigated. The effect of noise reduction and carrier signal suppression were confirmed by a circuit simulation based on the fabricated GMR element. A full-bridge circuit using four dual-spin-valve GMR elements was fabricated. It was confirmed that the carrier signal (2f components) was reduced to less than the signal level (f components). An increase in the noise level was found at the frequency range near f, which may be due to slight asymmetry shown in the magnetoresistance (MR) response.

Development of a high sensitive magnetic flux leakage instrument for imaging of localised flaws in small diameter ferromagnetic steel tubes

The authors report the development of a high sensitive, 16 channel magnetic flux leakage (MFL) instrument for imaging of localised flaws in small diameter ferromagnetic steel tubes. The instrument consists of a magnetising unit of two bobbin coils wound on a ferrite core, a sensing unit of flexible giant magneto-resistive (GMR) sensor array and an image visualisation unit. 3D-non-linear finite-element modelling has been carried out to optimise the inter-coil spacing of the bobbin coils and to identify the number of GMR elements and their locations between the bobbin coils. A series of experiments has been carried out to evaluate the flaw detection performance of the MFL instrument. Studies reveal that the instrument can image as small as 1.1 mm diameter hole and 0.46 mm deep shallow outer diameter circumferential notch in 17.2 mm diameter Mod. 9Cr-1Mo steel tubes used as steam generator material in prototype fast breeder reactor, with a signal-to-noise ratio better than 8 dB. Studies also indicate that intensity of MFL signal measured by the instrument increases with increase in hole diameter and notch depth. The instrument is portable and easy to use for field inspection of ferromagnetic steel components.

Novel Design of a Magnetically Switchable MOSFET using Magnetoresistive Elements

Various research activities have been carried out, individually, in the fields of MOSFET design andanalysis, and magnetoresistance; however, ourresearch focused on the design and analysis of a magnetically switchable MOSFET with the application of magnetoresistive elements. Theoretical study, calculations and simulations were used in order to design and analyze the magnetically switchable MOSFET. It was observed that the magnetoresistance values of 42%, 81% and 95%, respectively, for giant magnetoresistive element, tunneling magnetoresistive element and colossal magnetoresistive element resulted in reduced resistance values of 139.2Ω, 45.6Ω and 12Ω across the MOSFET in presence of magnetic field; as compared to a higher value of 240Ω in its absence. As a consequence, the gate-source voltage increased beyond the threshold value (1.5V), and the MOSFET switched ON. Accordingly, a magnetically switchable MOSFET was designed and its behavioural characteristics were analyzed.

Giant magnetoresistance in perpendicularly magnetized synthetic antiferromagnetic coupling with Ir spacer

Perpendicularly magnetized magnetic tunnel junction (p-MTJ) is a key element for developing high-density spin-transfer torque switching magnetoresistive random access memory. Recently, a large exchange coupling (IEC) in the synthetic antiferromagnetic reference layer with Ir interlayer was observed in p-MTJs. The evaluation of the IEC is, however, difficult due to the electrostatic breakdown of MTJs. This study demonstrates the evaluation of the IEC with Ir interlayer in giant magnetoresistive (GMR) nanopillar. We fabricated three kinds of perpendicularly magnetized GMR elements; bottom-free structures with Cu or Ir spacer, and top-free structure with Ir spacer. The magnetoresistance (RH) loops of all samples show sharp changes of the magnetoresistance at the magnetic fields over ±10 kOe, indicating the existence of the large IECs. In particular, a sharp change of the magnetoresistance at the field over ±20 kOe was found for the element with Cu of 2 nm thickness.

Simple linearising front‐end‐circuit for giant magneto‐resistance sensors

A simple and efficient linearisation front-end circuit for giant magneto-resistance (GMR) sensors are presented. This circuit essentially uses only a single opamp to linearise the (non-linear) GMR sensor output. A novel methodology that places an active GMR element, present inside the GMR sensor, in the feedback loop of the opamp is employed for linearisation. Initial validation of the front-end is performed using simulation and emulation studies. Then, the front-end, fabricated on a printed-circuit board, is interfaced with a GMR sensor. Experimentation showed output non-linearity is around 0.6%. The proposed front-end possess merits, like, low component-count and minimal error, no requirement of expensive amplifiers or matched resistors for linearisation, fast response, over the existing methods.

Miniature Tri-Axis Magnetometer With In-Plane GMR Sensors

A miniature tri-axis magnetometer with in-plane giant magnetoresistance (GMR) sensors and a fluxguide was designed, implemented, and characterized in this paper. Three GMR sensor dies were aligned on a printed circuit board with their sensing directions in the same plane. By using the flux bending effect of a fluxguide, the magnetic field components in the mounting plane of GMR sensors were generated, and hence, the effective sensing directions are not in the same plane. The tilt of effective sensing direction from the mounting plane allows the device to detect the out-of-plane field component. When the device was used as an electronic compass, the digitally demodulated sensor outputs were converted into the orthogonal geomagnetic field components by using a real-time conversion process with the voltage-to-field transfer matrix. The result shows that the device is suitable for compass application. Further works to implement the similar design on a CMOS chip with integrated GMR elements and a planar modulation coil will realize a compact and easy-to-fabricate tri-axis magnetic field sensor.

Design and performance evaluation of two novel linearisation circuits for giant magneto‐resistance based sensors

This study presents two simple and efficient linearisation circuits for giant magneto-resistance (GMR)-based magnetic field sensors. GMR sensors are commonly available in wheatstone-bridge form, comprising two active GMR and two passive GMR elements. The output of such a sensor possesses a non-linear dependence on the input magnetic field. The proposed linearisation circuits operate on the output of a GMR sensor and provide a linear output with respect to the magnetic field. The first GMR linearisation circuit (GLC1) is based on an enhanced feedback compensation approach, while the second (GLC2) scheme uses a constant current technique. The methodologies of the schemes are described using mathematical derivations. Detailed analyses of the schemes are carried out to bring out the effects of circuit and sensor non-idealities on circuit performance. Further, the circuits were implemented on printed circuit boards and tested. Test results showed the capability of GLC1 and GLC2 to produce linear transfer characteristics. A prototype GMR sensor unit was then fabricated and tested with the developed circuits. Output non-linearity obtained during the experimentation was around 0.7%. Analyses of the results proved the superior performance of GLC1 and GLC2 over the existing schemes.

Detection and Distinction of Conductive and Magnetic Security Markers Using Eddy-Current Inspection

A printed magnetic marker is used effectively on bills or check for security. In securing of valuable documents, a further improvement in the degree of security is always indispensable. The addition of a conductive marker as well as a magnetic marker enhances the value of information and the reliability of these markers. We applied an eddy-current testing (ECT) technique by using the micro ECT probe with a giant magneto-resistance element (GMR) to detect both markers. Two markers can be detected and also distinguished by recognizing both the amplitude and phase of the ECT signal. This paper describes the method of detection and the results for these markers printed by a magnetic and conductive ink.

Direct transfer of magnetic sensor devices to elastomeric supports for stretchable electronics.

A novel fabrication method for stretchable magnetoresistive sensors is introduced, which allows the transfer of a complex microsensor systems prepared on common rigid donor substrates to prestretched elastomeric membranes in a single step. This direct transfer printing method boosts the fabrication potential of stretchable magnetoelectronics in terms of miniaturization and level of complexity, and provides strain-invariant sensors up to 30% tensile deformation.

Magnetocardiography with GMR-based sensors

We have developed a sensor based on the Giant Magneto-Resistive (GMR) effect, associated to a superconducting flux-to-field transformer, which allows femtotesla field detection in a wide range of frequencies [1]. Such sensors are good candidates for measurements of biomagnetic signals generated by the brain or the heart. Here we present Magneto-Cardiographic (MCG) recordings over the chest of healthy volunteers at 4K and 77K with these sensors in a magnetically shielded room. The sensitivity is now limited by the 1/f noise of the GMR element which appears below few kHz. We present a technique based on supercurrent switching which reduces this low frequency noise.

A giant magnetoresistance ring-sensor based microsystem for magnetic bead manipulation and detection

In this paper a novel spin valve giant magnetoresistance (GMR) ring-sensor integrated with a microstructure is proposed for concentrating, trapping, and detecting superparamagnetic beads (SPBs). Taking advantage of the fact that SPBs can be manipulated by an external magnetic field, a unique arrangement of conducting microrings is utilized to manipulate the SPBs toward the GMR sensing area in order to increase the reliability of detection. The microrings are arranged and activated in such a manner so as to enable the detection of minute concentrations of SPBs in a sample. Precise manipulation is achieved by applying current sequentially to the microrings. The fabricated ring-shaped GMR element is located underneath the innermost ring and has a magnetoresistance of approximately 5.9%. By the performed experiments it was shown that SPBs could be successfully manipulated toward the GMR sensing zone.

High sensitivity giant magnetoresistance magnetic sensor using oscillatory domain wall displacement

A high sensitivity giant magnetoresistance (GMR) magnetic sensor has been demonstrated using the oscillatory domain wall displacing method. This sensor consists of a 30-μm-wide GMR element, CoFeB (10 nm)/Cu (2.2 nm)/CoFe (3 nm)/MnIr (10 nm), and a 100-μm-wide aluminum conducting wire placed onto the GMR element with an insulation layer between them. Domain walls in the free layer, whose easy axis is perpendicular to the length direction of GMR element, are oscillation-driven by a 140 kHz alternating magnetic field produced by an ac flowing through the aluminum conducting wire. The oscillatory wall displacement reduces the influence of wall coercivity and enables an output proportional to the external field. By applying a 1-kHz external field and separating the 1-kHz component from the amplified bridge signal using a band-pass filter, we successfully detected a magnetic field as small as 0.21 mOe (21 nT), which is much smaller than the wall coercivity of the free layer.

Theoretical description of a responsive magneto-hydrogel transduction principle

A theoretical description and analysis of a novel responsive hydrogel transduction principle is given. It enables a very accurate measurement of the volume change of a stimulus-responsive hydrogel and might be used as an in-body sensor system. The hydrogel contains anchored magnetic nanoparticles and is deposited onto a substrate in which a GMR (giant magneto resistance) element is embedded. The sensor layout is such that a maximum signal is obtained when the hydrogel thickness is only 5 μm, enabling a fast sensor response. The GMR sensor is operated at 1 MHz to suppress noise. When the frequency dependence of the susceptibility is taken into account, the theory correctly describes the sensor response. To measure the response, a Fe 3O 4 nanoparticle dispersion having a susceptibility of 10 at 1 MHz has been used. The response curve of the sensor has been measured by varying the concentration of this dispersion. For a responsive hydrogel loaded with 1 vol% of these nanoparticles, an expansion of only 0.3‰ is theoretically still detectable. Experimental work with magneto-hydrogels to further validate this result will be performed in the near future.

Thermoelectrically Controlled Spin-Switch

The search for novel spintronic devices brings about new ways to control switching in magnetic thin-films. In this work we experimentally demonstrate a device based on thermoelectrically controlled exchange coupling. The read out signal from a giant magnetoresistance element is controlled by exchange coupling through a weakly ferromagnetic Ni-Cu alloy. This exchange coupling is shown to vary strongly with changes in temperature, and both internal Joule heating and external heating is used to demonstrate magnetic switching. The device shows no degradation upon thermal cycling. Ways to further optimize the device performance are discussed. Our experimental results show a new way to thermoelectrically control magnetic switching in multilayers.

Microwave Oscillations of the Giant Magnetoresistive Element in a Magnetic Field Perpendicular to the Plane

Applied voltage and magnetic field dependence of microwave oscillations have been studied for the giant magnetoresistive (GMR) element. In this experiment, a magnetic field is applied perpendicular to the plane of GMR films. It is found that the power spectral density (PSD) is less dependent on the direction of magnetic field and the resistance gap induced by the magnetic field corresponds to the rapid increase of PSD. Moreover, noise peaks are observed in a high frequency range (about 8 GHz or higher) when a magnetic field of more than 2000 Oe is applied. The peak frequencies shift to higher frequency as the magnitude of magnetic field increases. In contrast to a magnetic field, the peak frequencies and their widths are more susceptible to the direction of current.

Programmable Spintronic Logic Devices for Reconfigurable Computation and Beyond—History and Outlook

Non-volatile and programmable logic devices are desirable for future reconfigurable computer chip. Spintronic logic devices, which use both electron spin and charge, can satisfy these requirements. Moreover, small device size, low power consumption, fast switching speed and programmable on the fly make it the most promising candidate for the next generation logic devices. First, this paper reviews the different types of spintronic logic devices. Secondly, we review the proposal and experimental demonstration of the magnetic tunnel junction (MTJ) and giant magnetoresistance element (GMR) based logic devices with the focus the development of single MTJ logic device with programmable functions and a MTJ-based full-adder logic circuit. Thirdly, a new generation of spintronic logic devices based spin torque transfer switching mechanism (or so-called current-induced-magnetization switching) is introduced. The last part gives an outlook of future prospective of spintronic logic devices and challenges.